Development of a high sensitivity PET projection imaging system for detecting and visualizing breast tumors continued during this reporting period. A prototype detector pair was fabricated and made operational. Bench top testing of this system and preliminary studies in humans indicated that previous estimates of the effects of radioactivity outside the field of view of the system were inaccurate. Instead, out-of-field activity at the levels required by the experimental paradigm saturated each of the detectors, rendering their performance non-linear to changes in target activity. As a result, detector geometry and shielding were redesigned to reduce the effects of out-of-field activity and to make the system respond linearly. This new system is currently undergoing tests in human female subjects and is thus far performing successfully in the linear regime. The system is also being used to evaluate the in vivo dynamics of new tumor-seeking PET radiopharmaceuticals in a human tumor- nude mouse model to identify those compounds most likely to label tumors in humans with high affinity.